Re: [PATCH] KVM: VMX: Do not overwrite vcpu->srcu_idx in vmx_vcpu_reset

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2013-03-14 20:14, Marcelo Tosatti wrote:
> On Thu, Mar 14, 2013 at 05:00:04PM +0200, Gleb Natapov wrote:
>> On Thu, Mar 14, 2013 at 03:52:11PM +0100, Jan Kiszka wrote:
>>> vmx_vcpu_reset may now be called while already holding the srcu lock, so
>>> we may overwrite what was already saved there. Also, we lock and unlock
>>> in the same context, thus there was no need to save to the vcpu anyway.
>>>
>>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>>> ---
>>>
>>> Marcelo just suggested this as the simplest fix for the issue caused by
>>> the INIT/SIPI patch. Avoiding srcu lock for TSS handling might still be
>>> possible but more tricky.
>>>
>>>  arch/x86/kvm/vmx.c |    5 +++--
>>>  1 files changed, 3 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>> index 958ac3a..be5b1dc 100644
>>> --- a/arch/x86/kvm/vmx.c
>>> +++ b/arch/x86/kvm/vmx.c
>>> @@ -4117,6 +4117,7 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu)
>>>  {
>>>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
>>>  	u64 msr;
>>> +	int idx;
>>>  
>>>  	vmx->rmode.vm86_active = 0;
>>>  
>>> @@ -4190,9 +4191,9 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu)
>>>  		vmcs_write16(VIRTUAL_PROCESSOR_ID, vmx->vpid);
>>>  
>>>  	vmx->vcpu.arch.cr0 = X86_CR0_NW | X86_CR0_CD | X86_CR0_ET;
>>> -	vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
>>> +	idx = srcu_read_lock(&vcpu->kvm->srcu);
>>>  	vmx_set_cr0(&vmx->vcpu, kvm_read_cr0(vcpu)); /* enter rmode */
>>> -	srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
>>> +	srcu_read_unlock(&vcpu->kvm->srcu, idx);
>> vmx_set_cr0() does:
>>                 srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
>>                 vmx_set_tss_addr(vcpu->kvm, 0xfeffd000);
>>                 vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
>> So with this change the sequence will be:
>>
>> vcpu->srcu_idx = srcu_read_lock()
>>   idx = srcu_read_lock(&vcpu->kvm->srcu); 
>>    srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
>>    vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
>>   srcu_read_unlock(&vcpu->kvm->srcu, idx);
>> srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
>>
>> Not sure this is valid.
> 
> The lock/unlocks must be paired.

Did you find out more than what Paolo reported?

> 
> Pass parameters around to make that happen?

Or we save/restore srcu_idx when overwriting.

Jan

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